1. Field of the Invention
This invention relates to range measuring systems such as radar, optical range finders, both passive and active, and sonar ranging systems using acoustical energy. More particularly, the invention relates to implementations of such range measuring systems that create an image.
2. Description of the Background Art
The present state of the art basically comprises two major areas: time of flight measurement systems and trigonometric systems. The most common forms of radar, laser radar and sonar are all classified under the time of flight category, while CAT scanners (Computer Aided Tomography) and the range measuring ability of a human vision system are examples of trigonometric systems.
Time of flight systems make use of the finite and usually constant propagation speed of energy in various mediums. In the simplest form, a pulse of energy is launched into the medium in the direction of the object whose range is to be measured. This energy reflects off the object and returns to a receiver located near the transmitter. The time duration measured from the launching of the pulse to its reception is multiplied by the propagation speed of the energy in the medium. The result is then divided by two to give the range to the object. It is also possible to use a continuous energy beam that is modulated as a function of time to measure range in the same manner and, in fact, a pulse system represents one extreme of modulation.
Usually the transmitter in a time of flight system forms the energy into a beam so as to illuminate a spot on the object that has a small range variation relative to the distance from the transmitter to the object. The receiver is similarly focused to receive only energy from the area on the object at which the transmitter is radiating, thereby improving the signal to noise ratio at the receiver. The range measurement is the average range to the area on the object being measured. Further, the transmitted and received beams can be made to be coaxial. A significant advantage is therefore achieved in certain situations and constitutes a major distinction between time of flight systems and trigonometric systems.
The method used to measure the propagation time for the above system varies considerably depending on the accuracy desired and the intended use of the range measurement. The most common form of measurement is to use digital counting electronics. A clock is used to provide a reference time interval to a digital counter circuit. The counter is first cleared, then started when the pulse is transmitted and then stopped and its value read when the return pulse is received. The count is a measurement of the elapsed time that can then be converted into range. The signal to clear and start the counter is usually well defined since it is generated within the system. The signal to stop the counter, coming from the receiver, usually requires considerable processing.
In particular, signal strength variations of the received pulse, generated as a result of reflections of the transmitted energy off objects of varying reflectivity, are removed. Additional processing is performed to minimize other effects that degrade the measurement. It is important in a system of this type to be able to distinguish the shape of the received pulse so as to be able to make an accurate time measurement. Ultimately, the accuracy of the timing measurements results in a minimum range resolution for the system. It is also common in this type of system to limit the ranges over which the measurements are made. Basically, the time measurement system only measures the time to pulses which return within a specific time interval. This results in the system having a minimum range and a maximum range, the range difference is referred to as the range depth over which the system can make measurements. Usually provision is made to move this range depth interval back and forth in range so as to allow the examination of a greater total range interval while having a small instantaneous range depth.
The above system measures range only to one point on an object at a time. To create a range image, a scanning system is usually employed to sweep the transmit and receive beams over one or two dimensions. An implementation such as this is referred to as a serial scanner. The resultant range image is a one or two dimensional array of data. The location of the data in the array is determined by the relative direction in which the range measurement was made. The direction is specified as an angular offset from the boresight of the system. The data at a specific location is the range (or a function of range) to the object in that direction. The data is quantized by the range resolution of the system and usually any single image is limited in maximum and minimum value by the range depth of the system. The individual range measurement values in the image are referred to as image elements.
A primary advantage of serial scanned systems is low cost. Only one transmitter and one receive and one time measurement system is used along with the scanning system. The major disadvantage is that to increase the number of points being measured requires a longer time to make the total image, since each individual measurement requires a certain minimum time. In addition, since different parts of the image are measured at different times, movement of the system relative to the objects being measured, or vice-versa, can create undesired effects. By scanning multiple transmitter/receiver pairs, or multiple receivers per transmitter, these effects can be reduced and the time to create an image decreased, but at increased cost. Also, by using multiple receivers it may be possible that the array of receivers can cover the entire area desired, removing the need for a scanner. In this case, the transmitter needs to cover the entire area. Multiple receivers require multiple time measurement systems and a means of obtaining data from each timer.
A variation of a multiple receiver system that operates in the optical spectrum is the range gated camera, such as is disclosed in U.S. Pat. No. 3,380,358 to Neumann. The camera, although it is a single device physically, is an example of a multiple receiver since it measures the light coming from many different parts of the scene in parallel. In this implementation, a pulse transmitter consisting usually of some form of laser is combined with the camera and an optical gating device in the form of a gated image intensifier. The intensifier is placed on the front of the camera so as to be able to gate the light returned from the object the laser illuminates. Timing electronics synchronize the camera, the intensifier gate and the laser pulse.
In operation, the intensifier is held in an OFF state during and for some predetermined time after the laser sends out a pulse of light. At the end of this time period, the intensifier is gated ON for a short time, exposing the television camera in a manner similar to a photographic camera. The camera then outputs this image. The image is a binary measurement of a specific range and range resolution as determined by the delay time between the sending of the light pulse and the gating of the intensifier, the velocity of light and the duration of the transmitted pulse and the camera gate duration. If an object was located at a range within the range resolution interval being examined, then there will be a nonzero output from the camera at the points in the image where the object was found. If there was no object at the specified range, only a low noise level will be output. Even though objects within the range resolution interval produce an image similar to a television image, this intensity modulation cannot be used in a range gated camera to determine the range to the individual points on the objects within the range resolution interval. The output is only a binary YES, there is something within this range interval, or NO, there is nothing there. By making successive measurements at differing ranges it is possible to build a range image from this binary data.
The major advantage of the gated camera system is that even though it creates a measurement at only one range resolution interval, it makes this measurement in parallel for all the individual image elements of the television camera. Therefore, an increase in the angular resolution of the camera can be made by adding more picture elements, but this does not incur additional time to build the image as in a serial scanner.
The disadvantage of this system is that many images must be made at successive ranges to build a range image with any significant range resolution within the range depth interval. This incurs an increasing time penalty to build a higher range resolution images. In making these successive measurements, this type of system will also suffer from the problem of relative movement between the camera and the objects being imaged, similar to a serial scanner.
Trigonometric systems determine range by measuring the angles to the same point on an object from two locations separated by a known baseline distance. In effect, a triangle is determined where two angles and a side are known, thus the remaining side can be determined although typically the perpendicular distance from the baseline to the opposite apex is computed.
In one configuration, an imaging trigonometric system is comprised of two television cameras which are separated by a baseline. The imagery from each camera is then processed to identify identical points in both images. The location, or angular displacement, of the points in the respective images can then be used in conjunction with the known baseline distance to compute the range to the point. Another configuration locates a transmitter on one end of the baseline which radiates the object to be measured with a reference beam. At the opposite end of the baseline, a receiver is located which can measure the angular position on the object where the reference beam is located.
The main disadvantage of these systems is that the processing to find identical points in both images is computationally intensive and prone to error due to the fact that the same point in both images may never match exactly since both images were made from different directions. Low contrast in the images also makes the problem of locating identical points difficult to impossible. In addition to the range accuracy being dependent on how well the identical point was located in the individual images, the angular resolution of the images, and the baseline separation distance verses the range also effect the accuracy. In many applications, the fact that a trigonometric system requires a baseline separation precludes its use.
Therefore, it is an object of this invention to provide an apparatus which overcomes the aforementioned inadequacies of the prior art devices and provides an improvement which is a significant contribution to the advancement of the range imaging art.
Another object of this invention is to provide a range imaging sensor for creating images containing range depth information.
Another object of this invention is to provide a range imaging sensor including a transmitter for transmitting energy toward the object to be ranged, an integrating receiver for receiving reflected energy from the object and integrating such energy, and a timing system for controlling the transmitter and the receiver such that the receiver integrates more energy from nearer objects than farther objects thereby creating a composite image in the receiver that varies as a function of range and unwanted information.
Another object of this invention is to provide a range imaging sensor including transmitter and receiver means to produce a composite image containing range information along with unwanted information and to produce a reference image containing substantially only unwanted information and including processor means for dividing the composite image by the reference image to cancel variations in the composite image not related to range, thereby computing a range image substantially representative of range to the objects.
Another object of this invention is to provide a range imaging sensor including transmitter and receiver means to produce an additive image representative of extraneous energy or energy originated from the objects from sources other than the transmitter, that can be subtracted from the composite and reference images thereby improving image quality.
The foregoing has outlined some of the more pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.